This invention relates to a can end and a method of manufacture of such a can end. In particular, it relates to a can end which has improved performance characteristics.
Containers such as cans which are used for the packaging beverages, for example, may contain a carbonated beverage which is at a higher than atmospheric pressure. Can end design has been developed to withstand this “positive” buckle pressure (sometimes also referred to as “peaking” pressure) up to defined minimum values (currently 90 psi for carbonated soft drinks) under normal operating conditions before failure. About 8 to 10 psi above this value, failure of conventional can ends involves loss of the circular profile and buckling of the end which, ultimately, leads to eversion of the end profile. Abuse conditions may also arise when a container is dropped or distorted, or when the product within the container undergoes thermal processing.
One solution to the problem of loss of circular profile is provided by the can end which is described in our U.S. Pat. No. 6,065,634. The can end shell (that is, the unseamed can end) of that patent includes a peripheral curl, a seaming panel, a chuck wall at an angle of between 30° and 60°, a narrow anti-peaking bead and a center panel. During seaming of the shell to the can body, the chuck wall is deformed at its upper end by contact with an anvil portion of the seaming chuck. The resulting profile provides a very strong double seam since the annulus formed by the seam has very high hoop strength and will resist distortion from its circular profile when subjected to thermal processing or when packaging carbonated beverages.
Stiffness is also provided to the beverage can end by the anti-peaking or countersink bead. This is an outwardly concave bead comprising inner and outer walls, joined by a curved portion. In the '634 patent this bead has walls which are substantially upright, although either may vary by up to +/−15°. This patent uses a small base radius (best fit) for the bead, typically 0.75 mm or less.
It is known from U.S. Pat. No. 6,089,072 that the width of the anti-peaking bead can be reduced by free drawing of the inner wall of the bead. This latter method avoids undue thinning of the bead as it is reworked. The resultant narrower bead optimises the stiffness of the can and, consequently, its resistance to buckling when attached to a can body having high internal pressure in the can.
Can ends such as those described in the above patents have high hoop strength and/or improved buckle performance such that they resist deformation when subjected to high internal pressure. In particular, the buckle pressure of the end of the '634 patent is well above the 90 psi can making industry minimum standard.
Whilst high hoop strength is predominantly beneficial it will affect the manner in which the can end ultimately fails. In a conventional can end, the circular periphery of the can end will tend to distort and become oval under high internal pressure. If the circular shape of the seamed end is free to distort to an oval shape under high internal pressure, as is usual, then part of the anti-peaking bead will open out along an arc at one end of the long axis of the oval shape as the can end events locally.
However, as the inventors have observed in the can end of the '634 patent in particular, it has been found that the stiff annulus formed by the double seam resists such distortion. As a result, when subjected to severe abuse conditions, dropping during transport, mishandling by machinery, freezing etc, it has been found that the resultant failure mode may lead to leakage of can contents. When distortion of the seam or anti-peaking bead is resisted by a strong seam and/or anti-peaking bead, failure can be by eversion of the bead at a single point rather than along an arc. Such point eversion leads to pin hole leaks or even splitting of the can end due to the localised fatiguing of the metal and extreme conditions may even be explosive